1. Field of the Invention
This invention relates to single lens reflex cameras, and more particularly to a focusing screen unit for a single lens reflex camera provided with an automatic sharpest focus detecting device.
2. Description of the Prior Art
To focus a single lens reflex (SLR) camera an operator normally looks through a viewfinder to observe an image of an object formed by an objective lens on a focusing screen. He then relies upon his own eyesight to determine whether the object is in focus. This visual and manual focusing procedure with the assistance of the focusing screen alone is, however, very troublesome, as is generally perceived by most beginning users of SLR cameras. A beginner requires considerable time and experience before being able consistently and accurately to adjust the objective lens to its optimum focus position. It is, therefore, exceedingly difficult for the average amateur to focus quickly without sacrificing the required level of accuracy. Thus, the photographer is obliged to proceed to take a picture although not always fully satisfied with the focus obtained.
Many attempts have been made to eliminate this inconvenience by providing a camera as equipped with an autofocus mechanism. Up to the present time, however, an autofocus mechanism has not found wide use in SLR cameras as compared with an automatic exposure control mechanism because of the following factors. First of all, the internal mechanisms of the camera require relatively large modifications for use thereof. The autofocus mechanism itself is so complicated in structure and so large in bulk and weight that it is in practice undesirable for the camera to be manufactured with such an additional mechanism. The production cost of the individual camera with autofocus capability is very high. Finally, the accuracy of a conventional autofocus mechanism for detection of the sharpest focus of the objective lens is not fully satisfactory.
To obtain the sharpest focus detecting device for a SLR camera in which the focusing of the objective lens corresponds to the degree of sharpness of an image of an object at the focal plane of the camera, the most expedient and simplest method involves an "electric eye" in lieu of the human eye. Specifically, it involves photoelectric element capable of responding to the variation in sharpness of the image for automatically detecting the point of sharpest focus. Essentially this photoelectric element must be arranged to receive at least part of a real image of the object formed by the objective lens or otherwise of a real image at least equivalent thereto, and the image receiving surface of the photoelectric element must be large enough to assure a sufficient accuracy despite variations in the photographic situations which may be encountered. However, the practical application of this method gives rise to a number problems with respect to the optical aspects of the arrangement and construction of the photoelectric element.
For example, the photoelectric element must be obtained from an image-forming light beam but positioned at a suitable location within the camera housing in cooperation with supplementary optical means arranged to direct a fraction of the image-forming light beam to the photoelectric element. Alternatively, an additional image forming lens mechanically linked with the objective lens may be used to form an image of the same object on the photoelectric element, while maintaining the sharpness of this image equivalent to that of the image formed on the imaging plane of the objective lens. Such a focus detecting optical system can not be provided without a relatively large modification of parts of the camera structure and is not favorable from the standpoint of an available spare space within the camera housing. What is worse is that, in the case of the first-mentioned arrangement of the photoelectric element, the image sharpness must be sensed at such low light levels or with such small images that the output signal of sufficiently high level is not available to assure good accuracy of detection of best focus, and, in the case of the second-mentioned arrangement, it is very difficult to achieve maintenance of optically precise coincidence in sharpness between the images formed by the objective lens and the additional image-forming lens throughout the entire focusing range. This introduces large errors into the focusing of the objective lens because the output signal of the photoelectric element does not correctly represent the degree of sharpness of an image at the focal plane of the camera. All of these problems may be considered as arising from the severe limitation on the position which the photoelectric element is intended to occupy.
In the field of SLR cameras, therefore, most photographers enjoy the fruits of the recently accelerated rates of exposure control automation, but are obliged to operate with old conventional visual and manual focusing procedures which rely solely upon the critical judgement of the naked eye. This conventional focusing procedure is associated with the above mentioned major inconveniences which have prejudiced the usefulness of automatic exposure control systems. Such systems were developed with the hope of obtaining easily operable snap photography and the like. Aside from such objective, it is highly desirable to enable the photographer who may be an average amateur user always to obtain obtain an optimum focus accurately and quickly because the focusing procedure is no longer hampered by his personal experience and insight. At the same time one wishes to preserve the essential parts of the internal camera structure and prevent changes which might result from the introduction of an automatic optimum focus detecting device into the SLR camera. Those changes might occur because of the fact that the availability of a position which the photosensitive element is intended to occupy is largely limited. The resultant complete SLR camera may be amenable to relatively low unit cost production techniques.